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Author name code: fischer
ADS astronomy entries on 2022-09-14
=author:"Fischer, C.E."
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Title: Towards the Identification and Classification of Solar
Granulation Structures Using Semantic Segmentation
Authors: Díaz Castillo, S. M.; Asensio Ramos, A.; Fischer, C. E.;
Berdyugina, S. V.
2022FrASS...9.6632D Altcode:
Solar granulation is the visible signature of convective cells at
the solar surface. The granulation cellular pattern observed in the
continuum intensity images is characterised by diverse structures e.g.,
bright individual granules of hot rising gas or dark intergranular
lanes. Recently, the access to new instrumentation capabilities has
given us the possibility to obtain high-resolution images, which have
revealed the overwhelming complexity of granulation (e.g., exploding
granules and granular lanes). In that sense, any research focused
on understanding solar small-scale phenomena on the solar surface
is sustained on the effective identification and localization of the
different resolved structures. In this work, we present the initial
results of a proposed classification model of solar granulation
structures based on neural semantic segmentation. We inspect the
ability of the U-net architecture, a convolutional neural network
initially proposed for biomedical image segmentation, to be applied to
the dense segmentation of solar granulation. We use continuum intensity
maps of the IMaX instrument onboard the Sunrise I balloon-borne solar
observatory and their corresponding segmented maps as a training
set. The training data have been labeled using the multiple-level
technique (MLT) and also by hand. We performed several tests of the
performance and precision of this approach in order to evaluate
the versatility of the U-net architecture. We found an appealing
potential of the U-net architecture to identify cellular patterns
in solar granulation images reaching an average accuracy above 80%
in the initial training experiments.
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Title: Newly formed downflow lanes in exploding granules in the
solar photosphere
Authors: Ellwarth, M.; Fischer, C. E.; Vitas, N.; Schmiz, S.;
Schmidt, W.
2021A&A...653A..96E Altcode: 2021arXiv210700582E
Context. Exploding granules have drawn renewed interest because of
their interaction with the magnetic field (either emerging or already
present). Especially the newly forming downflow lanes developing in
their centre seem to be eligible candidates for the intensification
of magnetic fields. We analyse spectroscopic data from two different
instruments in order to study the intricate velocity pattern within
the newly forming downflow lanes in detail. <BR /> Aims: We aim to
examine general properties of a number of exploding granules, such
as their lifetime and extend. To gain a better understanding of the
formation process of the developing intergranular lane in exploding
granules, we study the temporal evolution and height dependence of the
line-of-sight velocities at their formation location. Additionally, we
search for evidence that exploding granules act as acoustic sources. <BR
/> Methods: We investigated the evolution of several exploding granules
using data taken with the Interferometric Bidimensional Spectrometer and
the Imaging Magnetograph eXperiment. Velocities for different heights
of the solar atmosphere were determined by computing bisectors of the
Fe I 6173.0 Å and the Fe I 5250.2 Å lines. We performed a wavelet
analysis to study the intensity and velocity oscillations within
and around exploding granules. We also compared our observational
findings with predictions of numerical simulations. <BR /> Results:
Exploding granules have significantly longer lifetimes (10 to 15 min)
than regular granules. Exploding granules larger than 3.8″ form an
independent intergranular lane during their decay phase, while smaller
granules usually fade away or disappear into the intergranular area
(we find only one exception of a smaller exploding granule that also
forms an intergranular lane). For all exploding granules that form a new
intergranular downflow lane, we find a temporal height-dependent shift
with respect to the maximum of the downflow velocity. Our suggestion
that this results from a complex atmospheric structure within the newly
forming downflow lane is supported by the comparison with synthesised
profiles inferred from the simulations. We found an enhanced wavelet
power with periods between 120 s to 190 s seen in the intensity and
velocity oscillations of high photospheric or chromospheric spectral
lines in the region of the dark core of an exploding granule.
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Title: Interaction of Magnetic Fields with a Vortex Tube at Solar
Subgranular Scale
Authors: Fischer, C. E.; Vigeesh, G.; Lindner, P.; Borrero, J. M.;
Calvo, F.; Steiner, O.
2020ApJ...903L..10F Altcode: 2020arXiv201005577F
Using high-resolution spectropolarimetric data recorded with the
Swedish 1 m Solar Telescope, we have identified several instances of
granular lanes traveling into granules. These are believed to be the
observational signature of underlying tubes of vortical flow with
their axis oriented parallel to the solar surface. Associated with
these horizontal vortex tubes, we detect in some cases a significant
signal in linear polarization, located at the trailing dark edge of
the granular lane. The linear polarization appears at a later stage of
the granular lane development, and is flanked by patches of circular
polarization. Stokes inversions show that the elongated patch of linear
polarization signal arises from the horizontal magnetic field aligned
with the granular lane. We analyze snapshots of a magnetohydrodynamic
numerical simulation and find cases in which the horizontal vortex
tube of the granular lane redistributes and transports the magnetic
field to the solar surface causing a polarimetric signature similar to
what is observed. We thus witness a mechanism capable of transporting
magnetic flux to the solar surface within granules. This mechanism is
probably an important component of the small-scale dynamo supposedly
acting at the solar surface and generating the quiet-Sun magnetic field.
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Title: Evolution of Stokes V area asymmetry related to a quiet Sun
cancellation observed with GRIS/IFU
Authors: Kaithakkal, A. J.; Borrero, J. M.; Fischer, C. E.;
Dominguez-Tagle, C.; Collados, M.
2020A&A...634A.131K Altcode: 2020arXiv200105465K
A quiet Sun magnetic flux cancellation event at the disk center was
recorded using the Integral Field Unit (IFU) mounted on the GREGOR
Infrared Spectrograph (GRIS). The GRIS instrument sampled the event
in the photospheric Si I 10827 Å spectral line. The cancellation was
preceded by a significant rise in line core intensity and excitation
temperature, which is inferred from Stokes inversions under local
thermodynamic equilibrium (LTE). The opposite polarity features
seem to undergo reconnection above the photosphere. We also found
that the border pixels neighboring the polarity inversion line
of one of the polarities exhibit a systematic variation of area
asymmetry. Area asymmetry peaks right after the line core intensity
enhancement and gradually declines thereafter. Analyzing Stokes
profiles recorded from either side of the polarity inversion line
could therefore potentially provide additional information on the
reconnection process related to magnetic flux cancellation. Further
analysis without assuming LTE will be required to fully characterize
this event. <P />Movie associated to Fig. 2 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201936941/olm">https://www.aanda.org</A>
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Title: Photospheric Magnetic Fields of the Trailing Sunspots in
Active Region NOAA 12396
Authors: Verma, M.; Balthasar, H.; Denker, C.; Böhm, F.; Fischer,
C. E.; Kuckein, C.; González Manrique, S. J.; Sobotka, M.; Bello
González, N.; Diercke, A.; Berkefeld, T.; Collados, M.; Feller, A.;
Hofmann, A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pastor Yabar,
A.; Rezaei, R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K.; Volkmer,
R.; von der Lühe, O.; Waldmann, T.
2019ASPC..526..291V Altcode: 2018arXiv180507752V
The solar magnetic field is responsible for all aspects of solar
activity. Sunspots are the main manifestation of the ensuing solar
activity. Combining high-resolution and synoptic observations has
the ambition to provide a comprehensive description of the sunspot
growth and decay processes. Active region NOAA 12396 emerged on 2015
August 3 and was observed three days later with the 1.5-meter GREGOR
solar telescope on 2015 August 6. High-resolution spectropolarimetric
data from the GREGOR Infrared Spectrograph (GRIS) are obtained in the
photospheric lines Si I λ1082.7 nm and Ca I λ1083.9 nm, together
with the chromospheric He I λ1083.0 nm triplet. These near-infrared
spectropolarimetric observations were complemented by synoptic
line-of-sight magnetograms and continuum images of the Helioseismic
and Magnetic Imager (HMI) and EUV images of the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamics Observatory (SDO).
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Title: Observations of solar small-scale magnetic flux-sheet emergence
Authors: Fischer, C. E.; Borrero, J. M.; Bello González, N.;
Kaithakkal, A. J.
2019A&A...622L..12F Altcode: 2019arXiv190105870F
<BR /> Aims: Two types of flux emergence were recently discovered
in numerical simulations: magnetic loops and magnetic sheet
emergence. While magnetic loop emergence has been documented well in
recent years using high-resolution full Stokes data from ground-based
telescopes as well as satellites, magnetic sheet emergence is
still an understudied process. We report here on the first clear
observational evidence of a magnetic sheet emergence and characterise
its development. <BR /> Methods: Full Stokes spectra from the Hinode
spectropolarimeter were inverted with the Stokes Inversion based on
Response functions (SIR) code to obtain solar atmospheric parameters
such as temperature, line-of-sight velocities, and full magnetic
field vector information. <BR /> Results: We analyse a magnetic
flux emergence event observed in the quiet-Sun internetwork. After a
large-scale appearance of linear polarisation, a magnetic sheet with
horizontal magnetic flux density of up to 194 Mx cm<SUP>-2</SUP>
hovers in the low photosphere spanning a region of 2-3 arcsec. The
magnetic field azimuth obtained through Stokes inversions clearly
shows an organised structure of transversal magnetic flux density
emerging. The granule below the magnetic flux sheet tears the
structure apart leaving the emerged flux to form several magnetic
loops at the edges of the granule. <BR /> Conclusions: A large amount
of flux with strong horizontal magnetic fields surfaces through
the interplay of buried magnetic flux and convective motions. The
magnetic flux emerges within 10 minutes and we find a longitudinal
magnetic flux at the foot points of the order of ∼10<SUP>18</SUP>
Mx. This is one to two orders of magnitude larger than what has been
reported for small-scale magnetic loops. The convective flows feed
the newly emerged flux into the pre-existing magnetic population on
a granular scale. <P />Movie attached to Fig. 5 is available at <A
href="https://www.aanda.org/10.1051/0004-6361/201834628/olm">https://www.aanda.org</A>
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Title: Flows along arch filaments observed in the GRIS `very fast
spectroscopic mode'
Authors: González Manrique, S. J.; Denker, C.; Kuckein, C.; Pastor
Yabar, A.; Collados, M.; Verma, M.; Balthasar, H.; Diercke, A.;
Fischer, C. E.; Gömöry, P.; Bello González, N.; Schlichenmaier,
R.; Cubas Armas, M.; Berkefeld, T.; Feller, A.; Hoch, S.; Hofmann,
A.; Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Schmidt, D.; Schmidt,
W.; Sigwarth, M.; Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude,
J.; Strassmeier, K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2017IAUS..327...28G Altcode: 2017arXiv170102206G
A new generation of solar instruments provides improved spectral,
spatial, and temporal resolution, thus facilitating a better
understanding of dynamic processes on the Sun. High-resolution
observations often reveal multiple-component spectral line profiles,
e.g., in the near-infrared He i 10830 Å triplet, which provides
information about the chromospheric velocity and magnetic fine
structure. We observed an emerging flux region, including two small
pores and an arch filament system, on 2015 April 17 with the `very
fast spectroscopic mode' of the GREGOR Infrared Spectrograph (GRIS)
situated at the 1.5-meter GREGOR solar telescope at Observatorio del
Teide, Tenerife, Spain. We discuss this method of obtaining fast (one
per minute) spectral scans of the solar surface and its potential to
follow dynamic processes on the Sun. We demonstrate the performance
of the `very fast spectroscopic mode' by tracking chromospheric
high-velocity features in the arch filament system.
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Title: Chromospheric impact of an exploding solar granule
Authors: Fischer, C. E.; Bello González, N.; Rezaei, R.
2017A&A...602L..12F Altcode: 2017arXiv170600770F
Context. Observations of multi-wavelength and therefore height-dependent
information following events throughout the solar atmosphere and
unambiguously assigning a relation between these rapidly evolving
layers are rare and difficult to obtain. Yet, they are crucial for our
understanding of the physical processes that couple the different
regimes in the solar atmosphere. <BR /> Aims: We characterize
the exploding granule event with simultaneous observations of
Hinode spectroplarimetric data in the solar photosphere and Hinode
broadband Ca II H images combined with Interface Region Imaging
Spectrograph (IRIS) slit spectra. We follow the evolution of an
exploding granule and its connectivity throughout the atmosphere and
analyze the dynamics of a magnetic element that has been affected
by the abnormal granule. <BR /> Methods: In addition to magnetic
flux maps we use a local correlation tracking method to infer the
horizontal velocity flows in the photosphere and apply a wavelet
analysis on several IRIS chromospheric emission features such as
Mg II k2v and Mg II k3 to detect oscillatory phenomena indicating
wave propagation. <BR /> Results: During the vigorous expansion of
the abnormal granule we detect radially outward horizontal flows,
causing, together with the horizontal flows from the surrounding
granules, the magnetic elements in the bordering intergranular lanes
to be squeezed and elongated. In reaction to the squeezing, we detect
a chromospheric intensity and velocity oscillation pulse which we
identify as an upward traveling hot shock front propagating clearly
through the IRIS spectral line diagnostics of Mg II h&k. <BR />
Conclusions: Exploding granules can trigger upward-propagating shock
fronts that dissipate in the chromosphere. <P />Movies associated
to Figs. A.1 and A.2 are available in electronic form at <A
href="http://www.aanda.org/10.1051/0004-6361/201731120/olm">http://www.aanda.org</A>
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Title: Flow and magnetic field properties in the trailing sunspots
of active region NOAA 12396
Authors: Verma, M.; Denker, C.; Böhm, F.; Balthasar, H.; Fischer,
C. E.; Kuckein, C.; Bello González, N.; Berkefeld, T.; Collados,
M.; Diercke, A.; Feller, A.; González Manrique, S. J.; Hofmann, A.;
Lagg, A.; Nicklas, H.; Orozco Suárez, D.; Pator Yabar, A.; Rezaei,
R.; Schlichenmaier, R.; Schmidt, D.; Schmidt, W.; Sigwarth, M.;
Sobotka, M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier,
K. G.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016AN....337.1090V Altcode:
Improved measurements of the photospheric and chromospheric
three-dimensional magnetic and flow fields are crucial for a precise
determination of the origin and evolution of active regions. We present
an illustrative sample of multi-instrument data acquired during a
two-week coordinated observing campaign in August 2015 involving,
among others, the GREGOR solar telescope (imaging and near-infrared
spectroscopy) and the space missions Solar Dynamics Observatory (SDO)
and Interface Region Imaging Spectrograph (IRIS). The observations
focused on the trailing part of active region NOAA 12396 with complex
polarity inversion lines and strong intrusions of opposite polarity
flux. The GREGOR Infrared Spectrograph (GRIS) provided Stokes IQUV
spectral profiles in the photospheric Si I λ1082.7 nm line, the
chromospheric He I λ1083.0 nm triplet, and the photospheric Ca I
λ1083.9 nm line. Carefully calibrated GRIS scans of the active region
provided maps of Doppler velocity and magnetic field at different
atmospheric heights. We compare quick-look maps with those obtained
with the “Stokes Inversions based on Response functions” (SIR)
code, which furnishes deeper insight into the magnetic properties
of the region. We find supporting evidence that newly emerging flux
and intruding opposite polarity flux are hampering the formation
of penumbrae, i.e., a penumbra fully surrounding a sunspot is only
expected after cessation of flux emergence in proximity to the sunspots.
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Title: Fitting peculiar spectral profiles in He I 10830Å absorption
features
Authors: González Manrique, S. J.; Kuckein, C.; Pastor Yabar, A.;
Collados, M.; Denker, C.; Fischer, C. E.; Gömöry, P.; Diercke, A.;
Bello González, N.; Schlichenmaier, R.; Balthasar, H.; Berkefeld, T.;
Feller, A.; Hoch, S.; Hofmann, A.; Kneer, F.; Lagg, A.; Nicklas, H.;
Orozco Suárez, D.; Schmidt, D.; Schmidt, W.; Sigwarth, M.; Sobotka,
M.; Solanki, S. K.; Soltau, D.; Staude, J.; Strassmeier, K. G.; Verma,
M.; Volkmer, R.; von der Lühe, O.; Waldmann, T.
2016AN....337.1057G Altcode: 2016arXiv160300679G
The new generation of solar instruments provides better
spectral, spatial, and temporal resolution for a better
understanding of the physical processes that take place on the
Sun. Multiple-component profiles are more commonly observed with these
instruments. Particularly, the He I 10830 Å triplet presents such
peculiar spectral profiles, which give information on the velocity
and magnetic fine structure of the upper chromosphere. The purpose
of this investigation is to describe a technique to efficiently fit
the two blended components of the He I 10830 Å triplet, which are
commonly observed when two atmospheric components are located within
the same resolution element. The observations used in this study were
taken on 2015 April 17 with the very fast spectroscopic mode of the
GREGOR Infrared Spectrograph (GRIS) attached to the 1.5-m GREGOR solar
telescope, located at the Observatorio del Teide, Tenerife, Spain. We
apply a double-Lorentzian fitting technique using Levenberg-Marquardt
least-squares minimization. This technique is very simple and much
faster than inversion codes. Line-of-sight Doppler velocities can
be inferred for a whole map of pixels within just a few minutes. Our
results show sub- and supersonic downflow velocities of up to 32 km
s<SUP>-1</SUP> for the fast component in the vicinity of footpoints of
filamentary structures. The slow component presents velocities close
to rest.
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Title: Quiet Sun Magnetic Field Evolution Observed with Hinode SOT
and IRIS
Authors: Fischer, C. E.; Bello González, N.; Rezaei, R.
2016ASPC..504...19F Altcode:
We study two physical processes that can be commonly observed in
the quiet sun and involve temporal evolution of the magnetic field:
convective collapse and flux cancellation. The aim is to investigate
the response of the chromosphere to the magnetic events in the
photosphere below. We have calibrated and aligned a co-spatial and
co-temporal 3 hour quiet sun time series observed with the Hinode
SOT (Solar Optical Telescope) and the IRIS (Interface Region Imaging
Spectrograph) satellites. Convective collapse events are identified in
the photosphere by inverting spectropolarimetric data and searching for
magnetic field intensification, preceded by a downflow and accompanied
by the development of a bright point in Ca II H images. We find a
corresponding downflow in the low chromosphere as deduced from IRIS
Mg II k and h spectra and an ensuing oscillatory velocity pattern. We
use magnetograms in the high photosphere to study pairs of magnetic
elements involved in flux cancellation and find an increase in the
entire quasi-continuum of the IRIS Mg II k and h spectrum following
the flux cancellation process and indicating a substantial energy
deposit into the lower atmosphere.
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Title: Coimbra Solar Physics Meeting: Ground-based Solar Observations
in the Space Instrumentation Era
Authors: Dorotovic, I.; Fischer, C. E.; Temmer, M.
2016ASPC..504.....D Altcode:
No abstract at ADS
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Title: Unusual Stokes V profiles during flaring activity of a
delta sunspot
Authors: Fischer, C. E.; Keller, C. U.; Snik, F.; Fletcher, L.;
Socas-Navarro, H.
2012A&A...547A..34F Altcode: 2012arXiv1209.0983F
<BR /> Aims: We analyze a set of full Stokes profile observations of
the flaring active region NOAA 10808. The region was recorded with
the Vector-Spectromagnetograph of the Synoptic Optical Long-term
Investigations of the Sun facility. The active region produced
several successive X-class flares between 19:00 UT and 24:00 UT on
September 13, 2005 and we aim to quantify transient and permanent
changes in the magnetic field and velocity field during one of the
flares, which has been fully captured. <BR /> Methods: The Stokes
profiles were inverted using the height-dependent inversion code
LILIA to analyze magnetic field vector changes at the flaring site. We
report multilobed asymmetric Stokes V profiles found in the δ-sunspot
umbra. We fit the asymmetric Stokes V profiles assuming an atmosphere
consisting of two components (SIR inversions) to interpret the profile
shape. The results are put in context with Michelson Doppler Imager
(MDI) magnetograms and reconstructed X-ray images from the Reuven
Ramaty High Energy Solar Spectroscopic Imager. <BR /> Results: We
obtain the magnetic field vector and find signs of restructuring
of the photospheric magnetic field during the flare close to the
polarity inversion line at the flaring site. At two locations in the
umbra we encounter strong fields (~3 kG), as inferred from the Stokes
I profiles, which, however, exhibit a low polarization signal. During
the flare we observe in addition asymmetric Stokes V profiles at one
of these sites. The asymmetric Stokes V profiles appear co-spatial
and co-temporal with a strong apparent polarity reversal observed
in MDI-magnetograms and a chromospheric hard X-ray source. The
two-component atmosphere fits of the asymmetric Stokes profiles
result in line-of-sight velocity differences in the range of ~12 km
s<SUP>-1</SUP> to 14 km s<SUP>-1</SUP> between the two components in
the photosphere. Another possibility is that local atmospheric heating
is causing the observed asymmetric Stokes V profile shape. In either
case our analysis shows that a very localized patch of ~5″ in the
photospheric umbra, co-spatial with a flare footpoint, exhibits a
subresolution fine structure.
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Title: Fast horizontal flows in a quiet sun MHD simulation and their
spectroscopic signatures
Authors: Vitas, N.; Fischer, C. E.; Vögler, A.; Keller, C. U.
2011A&A...532A.110V Altcode:
Numerical simulations of solar surface convection have predicted
the existence of supersonic horizontal flows in the photospheric
granulation. Recently, the detection of such flows in data from the
Hinode satellite was reported. We study supersonic granular flows in
detail to understand their signatures in spectral lines and to test
the observational detection method used to identify these flows in
the Hinode observations. We perform time-dependent 3D radiative MHD
numerical simulations and synthesize the Fe i 6302 Å spectral lines at
the resolution of the Hinode data for different viewing angles covering
the center-limb variation. There is very large variation in the detailed
shape of the emergent line profiles depending on the viewing angle and
the particular flow properties and orientation. At the full simulation
resolution the supersonic flows can even produce distinct satellite
lines. After smearing to the Hinode resolution sufficient signature
of supersonic motion remains. Our analysis shows that the detection
method used to analyze the Hinode data is indeed applicable. However,
the detection is very sensitive to ad hoc parameter choices and can
also misidentify supersonic flows.
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Title: Transient events in the solar photosphere at high spatial
and temporal resolution
Authors: Fischer, C. E.
2011PhDT.......134F Altcode:
The research presented in this thesis showcases some of the diverse
and abundant transient events seen in observed and simulated magnetic
and velocity ?elds in the solar photosphere. This thesis emphasizes
the use and understanding of inversion codes to infer the magnetic and
velocity ?elds from polarized spectral line pro?les. These techniques
were applied to quiet sun as well as active region observations and
simulations. The analysis of the small-scale magnetic ?eld evolution,
as presented in Chapters II and III, sheds light on the evolution
of the quiet sun magnetism. This brings us closer to understanding
its dynamics and interplay with the weakly magnetized gas, which is
suspected to be the root cause for the majority of the heat transfer
from the photosphere into the chromosphere and corona. The data set
studied in Chapter IV is intriguing as it shows the magnetic and
velocity ?eld evolution during a ?are with high temporal cadence. The
analysis of unusual Stokes pro?les connects the lower photosphere to
the dynamics in the chromosphere and corona. The last chapter, Chapter
V, shows that the comparison of highly realistic 3 D simulations with
observations can tell us more about the underlying physical mechanisms
of the observed spectral signatures. The study not only con?rms the
validity of the theoretical model, but also exposes the violent,
high-speed dynamics present in the photospheric granulation.
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Title: Observations of solar scattering polarization at high spatial
resolution
Authors: Snik, F.; de Wijn, A. G.; Ichimoto, K.; Fischer, C. E.;
Keller, C. U.; Lites, B. W.
2010A&A...519A..18S Altcode: 2010arXiv1005.5042S
Context. The weak, turbulent magnetic fields that supposedly
permeate most of the solar photosphere are difficult to observe,
because the Zeeman effect is virtually blind to them. The Hanle
effect, acting on the scattering polarization in suitable lines,
can in principle be used as a diagnostic for these fields. However,
the prediction that the majority of the weak, turbulent field resides
in intergranular lanes also poses significant challenges to scattering
polarization observations because high spatial resolution is usually
difficult to attain. <BR /> Aims: We aim to measure the difference
in scattering polarization between granules and intergranules. We
present the respective center-to-limb variations, which may serve as
input for future models. <BR /> Methods: We perform full Stokes filter
polarimetry at different solar limb positions with the CN band filter
of the Hinode-SOT Broadband Filter Imager, which represents the first
scattering polarization observations with sufficient spatial resolution
to discern the granulation. Hinode-SOT offers unprecedented spatial
resolution in combination with high polarimetric sensitivity. The CN
band is known to have a significant scattering polarization signal,
and is sensitive to the Hanle effect. We extend the instrumental
polarization calibration routine to the observing wavelength,
and correct for various systematic effects. <BR /> Results: The
scattering polarization for granules (i.e., regions brighter than
the median intensity of non-magnetic pixels) is significantly larger
than for intergranules. We derive that the intergranules (i.e., the
remaining non-magnetic pixels) exhibit (9.8±3.0)% less scattering
polarization for 0.2 < μ ≤ 0.3, although systematic effects cannot
be completely excluded. <BR /> Conclusions: These observations constrain
MHD models in combination with (polarized) radiative transfer in terms
of CN band line formation, radiation anisotropy, and magnetic fields.
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Title: Inversions of High-Cadence SOLIS-VSM Stokes Observations
Authors: Fischer, C. E.; Keller, C. U.; Snik, F.
2010ASSP...19..515F Altcode: 2010mcia.conf..515F
We have processed full-Stokes observations made with the SOLIS-VSM
using Fe I 630.15 and Fe I 630.25 nm. The data have high spectral and
temporal resolution, moderate spatial resolution, and large polarimetric
sensitivity and accuracy. We use the code LILIA, an LTE inversion code
written by Socas-Navarro (2001), in order to invert the data in vector
magnetic fields. The 180-degree ambiguity in magnetic field orientation
is solved by using the Non-Potential Field Calculation (NPFC) method
of Georgoulis (2005). The output product are maps of the fullmagnetic
field vector at the photospheric level, as illustrated in Fig. 1. We
performed such inversions for observations of active region NOAA 10808
taken during an X-class flare in September 2005. Details of the data
processing and the first results are given in the proceedings of the
Fifth Solar PolarizationWorkshop (ASP Conf. Ser., in press).
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Title: Statistics of Convective Collapse Events in the Photosphere
and Chromosphere Observed with the HINODE SOT
Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.;
Keller, C. U.
2009ASPC..415..127F Altcode:
Convective collapse, a theoretically predicted process that
intensifies existing weak magnetic fields in the solar atmosphere,
was first directly observed in a single event by Nagata et al. (2008)
using the high resolution Solar Optical Telescope (SOT) of the Hinode
satellite. Using the same space telescope, we observed 49 such events
and present a statistical analysis of convective collapse events. Our
data sets consist of high resolution time series of polarimetric
spectral scans of two iron lines formed in the lower photosphere and
filter images in Mg I b<SUB>2</SUB> and Ca II H. We were thus able
to study the implication of convective collapse events on the high
photospheric and the chromospheric layers. The physical parameters from
the full Stokes profiles were obtained with the MERLIN Milne-Eddington
inversion code. For each of the 49 events we determined the duration,
maximum photospheric downflow, and field strength increase. We found
event durations of about 10 minutes and field strengths of up to
1.65 kG.
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Title: Statistics of convective collapse events in the photosphere
and chromosphere observed with the Hinode SOT
Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.;
Keller, C. U.
2009A&A...504..583F Altcode: 2009arXiv0906.2308F
Convective collapse, a theoretically predicted process that intensifies
existing weak magnetic fields in the solar atmosphere, was first
directly observed in a single event by Nagata et al. (2008, ApJ,
677, L145) using the high resolution Solar Optical Telescope (SOT)
of the Hinode satellite. Using the same space telescope, we observed
49 such events and present a statistical analysis of convective
collapse events. Our data sets consist of high resolution time series
of polarimetric spectral scans of two iron lines formed in the lower
photosphere and filter images in Mg I b{2} and Ca II H, spectral lines
that are formed in the high photosphere and the lower chromosphere,
respectively. We were thus able to study the implication of convective
collapse events on the high photospheric and the chromospheric
layers. We found that in all cases, the event was accompanied by a
continuum bright point and nearly always by a brightening in the Ca
II H images. The magnesium dopplergram exhibits a strong downflow in
about three quarters of the events that took place within the field
of view of the magnesium dopplergram. The physical parameters from
the full Stokes profiles were obtained with the MERLIN Milne-Eddington
inversion code. For each of the 49 events we determined the duration,
maximum photospheric downflow, field strength increase and size. We
found event durations of about 10 min, magnetic element radii of about
0.43 arcsec and 0.35 arcsec, before and after the event, respectively,
and field strengths of up to 1.65 kG.
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Title: Vector Magnetic Field Inversions of High Cadence SOLIS-VSM Data
Authors: Fischer, C. E.; Keller, C. U.; Snik, F.
2009ASPC..405..311F Altcode:
We have processed full Stokes observations from the SOLIS VSM in the
photospheric lines Fe I 630.15 nm and 630.25 nm. The data sets have
high spectral and temporal resolution, moderate spatial resolution,
and large polarimetric sensitivity and accuracy. We used the LILIA, an
LTE code written by \citet{fischer_Navarro2001} to invert the data. We
also applied the non-potential magnetic field calculation method
of \citet{fischer_Manolis2005} in order to resolve the 180 degree
ambiguity. The output are maps of the full magnetic field vector at
the photospheric level. Here we present the first inversions of the
active region NOAA 10808 during an X-class flare, which occurred on
13 September 2005.